1. Field of the Invention
This invention relates to a liquid injection recording apparatus, and more particularly to a liquid injection recording apparatus having means for forming so-called droplets of recording liquid.
2. Description of the Prior Art
A recording head applied to a liquid injection recording apparatus is generally provided with minute liquid discharge ports (orifices), liquid flow paths, an energy acting portion provided in a portion of the liquid flow paths, and energy generating means generating droplet forming energy for acting on the liquid in the energy acting portion.
As the energy generating means, an electromechanical converting member such as a piezo element is used in the recording methods disclosed, for example, in U.S. Pat. No. 3,683,212 and U.S. Pat. No. 3,946,398, and an example using an electro-heat converting member as the energy generating means is described in one of the recording methods disclosed in Japanese Laid-open Patent Application No. 59936/1979 (corresponding DOLS 2843064 and U.S. Ser. No. 948,236). Also, in another recording method disclosed in this Japanese Laid-open Patent Application No. 59936/1979, there is described an example in which no special means is provided in the energy acting portion but an electromagnetic wave such as laser is applied to the energy acting portion and the liquid therein is caused to absorb the electromagnetic wave and generate heat and recording is accomplished with droplets being caused to be discharged and fly by the action of the heat generation, as it were, an example in which the liquid to which the electromagnetic wave is applied provides the energy generating means.
The above-described liquid injection recording methods are such that mechanical pressure, heat energy or electromagnetic energy is caused to act on the liquid in the energy acting portion to thereby obtain a motive force for discharge of the liquid, but to enhance the quality of recorded images and enable high-speed recording to be accomplished in such recording methods, it is necessary that discharge of droplets be executed stably and continuously repetitively by the recording head and that improvement of the droplet formation frequency (the number of droplets formed per unit time=the droplet formation frequency per unit time) of the recording head and stabilization of droplet formation characteristics be achieved.
In the past, however, all of these requirements could not be said to have been sufficiently met.
On the other hand, attention has recently been paid particularly to the on-demand type liquid injection recording system.
As a specific example of the on-demand type system, there is known a system which utilizes a heat-generating resistance member, known as an electro-heat converting member in the recording method described, for example, in the aforementioned Japanese Laid-open Patent Application No. 59936/1979, to heat the liquid in the pressure generating portion and impart to the liquid the pressure generated when the liquid is suddenly gasified, thereby accomplishing discharge of droplets. This system has a great advantage that because droplets can be discharged from orifices only when necessary for printing, means for collecting unnecessary liquid and means such as a high voltage source for deflection are unnecessary. However, this system is still left to be improved in the following point. That is, the discharge pressure for causing droplets to be discharged from the orifices is relatively low and the discharge of liquid may be delicately varied by the extraneous vibration relative to the recording head or by the unnecessary heat conduction from the electro-heat converting member or by mixing of dust or bubbles and it is sometimes difficult to continue stable discharge of droplets.
The recording head of a liquid injection recording apparatus of the construction as shown in the schematic perspective view of FIG. 1 of the accompanying drawings is heretofore known. In FIG. 1, reference numeral 101 designates droplets, reference numeral 102 denotes orifices, reference numeral 103 designates an orifice plate, reference numeral 104 denotes a base plate, reference numeral 105 designates electro-heat converting members, reference numeral 106 denotes liquid flow paths, reference numeral 107 designates a liquid supply path, and reference numeral 108 denotes heat acting portions. In the liquid injection recording apparatus of FIG. 1, liquid is supplied from the liquid supply path 107 to the liquid flow paths 106 and the liquid is discharged as droplets 101 from the liquid flow paths 106 through the orifices 102 by the electro-heat converting members 105 of the heat acting portions 108 in the liquid flow paths 106.
The inventors have found that such conditions as the shape of the orifices 102 and the thickness of the orifice plate 103 greatly affect the manner in which the discharged droplets 101 fly, in other words, the accuracy of the droplet discharge and the follow-up characteristic of the droplets for an input signal.
The shape of the openings according to the prior art will now be described by taking as an example the schematic fragmentary cross-sectional views as shown in FIGS. 2 to 4 of the accompanying drawings.
In FIGS. 2 to 4, reference numerals 202, 302 and 402 designate an orifice, reference numeral 203, 303 and 403 denote an orifice plate, reference numerals 204, 304 and 404 designate a base plate, reference numerals 205, 305 and 405 denote an electroheat converting member, and reference numerals 208, 308 and 408 designate a heat acting portion.
In the example shown in FIG. 2, the cross-sectional area S.sub.1 of the opening which is adjacent to the heat acting portion is equal to the minimum cross-sectional area S.sub.2 of the orifice (opening). The square roots of the cross-sectional area S.sub.1 and the minimum cross-sectional area S.sub.2 are represented by R and r, respectively. That is, if the average orifice diameter R=.sqroot.S.sub.1 and the minimum average orifice diameter R=.sqroot.S.sub.2, then R=r in the case of FIG. 2. Orifices of such a shape have heretofore often been used. However, the orifice of such a shape can accomplish relatively stable discharge of droplets while, on the other hand, it suffers from a problem that the resistance of droplet discharge is increased due to the thickness of the orifice plate 203 and the flying speed of discharged droplets is decreased. For example, if an attempt is made to effect recording by effecting high-speed scan by the use of a liquid injection recording apparatus having such an orifice shape, the droplet discharge speed is remarkably reduced as compared with the scan speed, and this may lead to cases where the variation in the scan speed cannot be absorbed. Accordingly, the accuracy with which droplets land on the recording medium is reduced to make it difficult to obtain excellent images.
FIG. 3 shows an example in which the diameter of the orifice 302 is not constant but the minimum average orifice diameter r on the atmosphere side is smaller than the average orifice diameter R on the heat acting portion 308 side (r&lt;R) and the orifice plate 304 is thin. Liquid injection recording apparatus having orifices of such a shape are also popular. However, in the case of such an orifice shape, the droplet discharge speed is increased due to the orifice plate 303 being thin, but in some cases, high stability of droplet discharge may not be obtained. Further, the use of such a thin orifice plate 303 may lead to the occurrence of a problem that air enters when droplets are discharged. Accordingly, again in this case, it cannot be expected to obtain excellent image recording stably and continuously.
Further, an example as shown in FIG. 4 wherein the average orifice diameter on the heat acting portion 408 side is increased toward the atmosphere side would also occur to mind, but again in this case, the droplet discharge speed and the droplet discharge direction are unstable and also, the introduction of gas from outside is intense. Accordingly, again in a liquid injection recording apparatus having such inverted tapered orifices, excellent image recording cannot be expected because stable discharge of droplets is not effected.
Of the orifice shapes of the liquid injection recording apparatuses as described above, the orifice shapes shown in FIGS. 2 and 4 can be formed by the use of photosensitive resin, for example, Permanent Photopolymer Coating RISTON Solder Mask 730S produced by Dupont, Inc. and through the photo-forming method, and the orifice shape shown in FIG. 3 can be formed by chemically etching stainless steel SUS-316.
As described above, even the orifice shapes heretofore generally used cannot actually provide a wide range of stable discharge of droplets, and this may sometimes lead to the occurrence of a problem in respect of excellent image recording.